Embryonic stem cells are stem cells established from the inner cell mass of mammalian blastocysts, and they can be proliferated infinitely while maintaining the ability to differentiate into all types of cells (differentiation pluripotency). This property anticipates stem cell therapy of myocardial infarction, Parkinson's disease patients, or such, which is achieved by transplanting myocardial cells or nerve cells induced and prepared in large quantities from ES cells. Furthermore, uses in basic pathological and pharmacological studies and as a development tool in drug discovery are also anticipated. However, these ES cells have the ethical problem of utilizing and sacrificing human fertilized eggs. There is also the problem of immune rejection where the histocompatibility antigens of limited donor fertilized eggs do not match with the patient. On the other hand, tissue stem cells such as neural stem cells, hematopoietic stem cells, and mesenchymal stem cells are present in every tissue of the living body. Since tissue stem cells do not use fertilized eggs, there are few or no ethical problems, and since cells of the patients themselves can he used, immune rejection reactions can also be avoided. However, properties of tissue stem cells are not necessarily understood, and therefore they are difficult to isolate, and their numbers are also very few. Their proliferative ability and differentiation ability are also much more limited compared to ES cells. If somatic cells such as tissue stem cells and differentiated cells can be converted by some means into cells similar to ES cells having a high proliferative ability and differentiation pluripotency (referred to as ES-like cells), such ES-like cells will be ideal stem cells in clinical applications and such.
Specifically, cells of mammals, particularly somatic cells of patients (tissues of the skin, stomach or lung, blood cells, and such) are collected, and these cells are cultured and then stimulated with nuclear reprogramming factors (factors that induce nuclear reprogramming) to produce ES-like cells (they may also be called “artificial pluripotent stem cells”, “induced pluripotent stem cells (iPS cells)”, or “embryonic stem cell-like cells”). These produced cells are expected to be applied clinically as stem cells or used in basic research including pharmacological or pathological research (Patent Document 1) just as they are, or after storage in cell banks. Furthermore, experiments to confirm pharmaceutical effects can also be carried out using artificial pluripotent stem cells established from patients.
Examples of nuclear reprogramming factors include the Oct gene, the Klf gene, the Myc gene, the Sox gene, the Nanog gene, the Lin28 gene, the TERT gene, and the SV40 Large T gene (Patent Document 2, Non-Patent Documents 1 to 7).
For example, it is known that the above-mentioned ES-like cells can be produced from the above-mentioned somatic cells using the following four recombinant virus vectors (Non-Patent Document 1 to 7). When the produced ES-like cells described above are used clinically, they may be able to avoid problems of immune rejection and ethical problems.                (1) gamma retroviral vector or lentiviral vector (hereinafter, these vectors will be collectively referred to as “retroviral vectors”) containing the Oct3/4 gene        (2) retroviral vector containing the Klf4 gene        (3) retroviral vector containing the c-Myc gene        (4) retroviral vector containing the Sox2 gene        
The above-mentioned patent documents and non-patent documents are as follows: